Natural gas supply in North America is continually growing, mostly due to production of new shale gas, recent discoveries of offshore gas fields, and to a lesser extent, stranded natural gas brought to market after construction of the Alaska natural gas pipeline, and it is believed that shale gas and coal-bed methane will make up the majority of the future growth in the energy market.
While natural gas supply is increasing, crude oil supply is depleting as there are no significant new discoveries of oil reserves. If this trend were to continue, transportation fuel derived from crude oil will soon become cost prohibitive, and alternate renewable fuels (and particularly transportation fuels) are needed. Moreover, since combustion of natural gas also produces significantly less CO2 as compared to other fossil materials (e.g., coal or gasoline), use of natural gas is even more desirable. Natural gas used for transportation fuel must be in a denser form, either as CNG (compressed natural gas) or LNG. CNG is produced by compression of natural gas to very high pressures of about 3000 to 4000 psig. However, even at such pressures, the density of CNG is relatively low and storage at high pressure requires heavy weight vessels and is a potential hazard. On the other hand, LNG has a significantly higher density and can be stored at relatively low pressures of about 20 to 150 psig. Still further, LNG is a safer fuel than CNG as it is at lower pressure and not combustible until it is vaporized and mixed with air in the proper ratio. Nevertheless, CNG is more common than LNG as a transportation fuel, mainly due to the high cost of liquefaction and the lack of infrastructure to support LNG fueling facilities.
LNG can be used to replace diesel and is presently used in many heavy duty vehicles, including refuse haulers, grocery delivery trucks, transit buses, and coal miner lifters. To increase the LNG fuel markets, small scale LNG plants must be constructed close to both pipelines and LNG consumers as long distance transfer of LNG is costly and therefore often not economical. Such small scale LNG plants should be designed to produce 30 tons to 130 tons per day of LNG by liquefying 2 to 10 MMscfd pipeline gas. Moreover, such small scale LNG plants must be simple in design, easy to operate, and sufficiently robust to support an unmanned operation. Still further, it would be desirable to integrate liquefaction with LNG truck fueling operations to allow for even greater delivery flexibility.
Various refrigeration processes are known in the art for LNG liquefaction. The most common of these refrigeration processes are the cascade process, the mixed refrigerant process, and the propane pre-cooled mixed refrigerant process. While these known methods are very energy efficient, such methods are often complex and require circulating several hydrocarbon refrigerants or mixed hydrocarbon refrigerants. Unfortunately, such refrigerants (e.g., propane, ethylene, and propylene) are explosive and hazardous in the event of leakage.
There are several recent innovations in LNG plant design. For example, U.S. Pat. No. 5,755,114 to Foglietta teaches a hybrid liquefaction cycle which includes a closed loop propane refrigeration cycle and a turboexpander cycle. Compared to conventional liquefaction processes, this process has been simplified, but is still unsuitable and/or economically unattractive for small scale LNG plants. U.S. Pat. No. 7,673,476 to Whitesell discloses a compact and modular liquefaction system that requires no external refrigeration. The system uses gas expansion by recycling feed gas to generate cooling. While this design is relatively compact, operation of the recycle system is complicated and the use of hydrocarbon gas for cooling remains a safety concern. U.S. Pat. No. 5,363,655 to Kikkawa teaches the use of gas expander and plate and fin heat exchangers for LNG liquefaction. While providing several advantages, such process is still too complex and costly for small scale LNG plants.
Further compounding the above noted drawbacks is the fact that most of the known systems lack the capability for integration of a small scale LNG plant with an LNG loading operation. Thus, the current practice for loading an LNG truck generally requires an LNG pump to pump the LNG from the storage tanks to the LNG trucks. Remarkably, the boil off vapors generated during the LNG truck loading operation are vented to the atmosphere which is a safety hazard and creates emission pollution.
Thus, while all or almost all of the known configurations and methods provide some advantages over previously known configurations, various disadvantages remain. Among other things, most of the known LNG liquefaction methods and configurations are complex and costly and hence unsuitable for the small scale LNG plants. In addition, most known plants lack an integrated system for LNG loading operations, which is highly desirable for small scale LNG plants.